Are pregnancy results reliable?
Yes. They are 99.9% accurate
Doesn't matter when you took the test, plan b caused a bleed and not a period but you ae not pregnant, for future info you wont get an accurate result before 14dpo as it takes that time for egg to get to the womb and implant there and hcg levels wont be high enough for positive result until after then. Implantation bleeding is rare but those that get it will be once or twice when they wipe and so light you cant really see it.
It's not implantation bleeding. That's a spot so small and light you'd miss it unless you obsessively examine toilet paper.